Production of black oxide films on aluminum



Patented June 22, 1954 PRODUCTION OF BLACK OXIDE FILMS ON ALUMINUltI George L. Deniston, Dayton, Ohio No Drawing. Application February 25, 1954, Serial No. 412,630

4 Claims. (01. 148-621) This invention relates to the chemical treatment of aluminum and aluminum alloy surfaces to produce black protective coatings, consisting in large part of aluminum oxide. Aluminum or aluminum alloys regardless of the purity of the metal or the nature of the alloying element, will be inclusively described by the generic term aluminum throughout this specification and the appended claims. The term protective oxide coating does not include the very thin film of aluminum oxide which is formed upon aluminum or any of its alloys by reason of contact with the atmosphere.

One of the standard methods of treating aluminum is by the electrolytic anodizing processes. In this process the aluminum articles are fastened on aluminum racks or fixtures, then immersed in a bath of the electrolyte, and an anodizing current passed therethrough. After a period of 10 to 60 minutes a film is formed or de posited on the parts which are submerged in the solution and which are conducting electric current. One of the disadvantages of this anodic treatment is the slowness of the batch type production of the so-called oxide film. Another A variety of methods have been proposed for producing protective oxide coating on aluminum by chemical treatment as by immersion in solutions containing ammonia plus metallic salts. These chemical processes also have their disadvantages. Some of these chemical treatments produce a soft film having a poor adherence as Well as only fair corrosion or abrasion resistance. Inasmuch as adherence is a primary factor because the function of an oxide film is to improve adherence of protective coatings such as paint, these processes have not been commercially successful. These chemical processes have also failed to gain acceptance because they failed to produce decorative or colored films on aluminum without the use of added coloring matter. In

come the disadvantages and shortcomings of the processes utilized heretofore in the production of oxide coatings on aluminum and aluminum alloys.

It is a further object to produce a black oxide like film on aluminum without the use of coloring agents such as dyes.

It is the object of this invention to provide a process that produces a hard, adherent aluminum oxide film in a matter of one minute compared to 15 minutes required previously.

It is still another object of this invention to provide a process by which aluminum and its alloys may have affixed upon their surface colors such as black, or shades therefore, which are uniform, serviceable and permanent in nature.

It is still a furuther object of the present invention to provide a method of producing a base for paint or similar protective finishes.

It is another object of this novel invention to provide a practical, commercial mass production, continuous method of forming on aluminum, webs or piece-parts, cast, wrought, spun or formed by other means, a dense, uniform adherent coating of protective oxide.

Another object of this invention is to provide a process which has a lower operating cost, due to its elimination of expensive equipment needed in the conventional electrolytic anodizing method.

It is another object of this invention to provide a process wherein irregular shaped aluminum piece-parts can be uniformly coated with an oxide film, something which cannot be done satisfactorily by conventional anodizing methods.

It is another object of this invention to provide a process wherein the piece-part may be fabricated and then immersed in the novel oxidizing bath to produce an oxide-like film thus eliminating the usual damage done by the conventional anodizing process, which must anodize a piece at a time and then assemble.

Other and more specific objects and advantages will be apparent to one skilled in the art as the following description proceeds.

In the present invention aluminum objects are immersed in a hot aqueous solution characterized by the presence of sulfate ions in combination with chromate ions, chloride ions, permanganate ions, and a wetting agent of the sulfonate group, all of which ingredients are present in definite relative proportions.

Not only must the ingredients be present in the proper proportions but the ingredients must It be added in a proper sequence to avoid developing a spotty and thin oxide coating.

Aluminum that has been etched mechanically, electrolytically, or chemically, buffed or polished by mechanical, chemical or electrical means, as well as aluminum furnished by the mill, can be treated by this novel process. Wrought castings produced by the conventional means, spun, forged or extruded aluminum or its alloys may be treated by this invention.

It is commonly known that all types of aluminum have an oxide film formed by the oxidizing action of the oxygen in the air. The process of this invention removes the natural oxide by the simple immersion of the aluminum parts into the solution and then produces the black film described in this invention.

The available oxygen derived from the reaction of the hot chromate ions in the acid media, principally sulphuric, produces the protective films when the aluminum part is immersed into the solution. The chromate ions in the acid bath produce a part of the oxidizing action and also form acids of the chromium element which assist in the other reactions. Chromate ions may be derived from chromic acids, as well as from Water soluble chromate or dichromate salts such as the alkali metal, magnesium, or ammonium salts. The percent of the chromic acid (ClOs) must be held within relatively narrow limits.

Sulphuric acid is one of the main acid constituents and is essential along with the other acids to produce the protective films.

The following examples serve to illustrate my invention without limiting it to the specific details given therein:

Chart Sodium Chromate, NazCrOr.

Sulphuric Acid, HzSO4 Hydrochloric Acid, H01

Potassium Permanganate, KMnOr.

Wetting Agent, Sodium salt of Alkyl Aryl Sulfonates.

Ratio of Amount of solution to Amount of Water.

ated by the action of the oxidizing agents upon hydrochloric acid, to be available to assist in the formation of a more dense black coating. Oxygen is liberated due to the formation of unstable hypochlorous acid which decomposes back to the stable acid and oxygen. Side reactions consist of formation of chlorites under the above conditions. This black film is composed substantially of oxides and complex hydrates and salts of aluminum.

The sodium chromate-sulphuric acid combination is augmented by potassium permanganate. These components produce a part of the oxygen which combines with the nascent aluminum to produce this black oxide-like protective film. The presence of this additional oxidizing agent is essential to concentrate the oxidizing action in order to produce the dense black film.

A wetting agent compatible with strong acid solutions is very essential for formation of the black film produced by the above solutions. The

wetting agent must be able to withstand the low pH of the acid solutions and not decompose too rapidly. The sulfonate type wetting agents produces an intimate wetting of the base aluminum and its natural oxide so as to allow the active ingredients of this invention to perform their desired functions. If the proper wetting agent is not used, unsatisfactory spotty films are formed. Naccanol NR, an anionic synthetic organic detergent of alkyl aryl sulfonate type has been found to fulfill these requirements. This material is produced by National Aniline Division of Allied Chemical and Dye Corp.

The satisfactory workable limits of this invention are fairly narrow to secure proper results.

Limits Per cent Sodium chromate 26.0 to 30.6 Sulphuric acid 9.4 to 22.4 Hydrochloric acid 14.8 to 34.6 Potassium permanganate 26.5 to 30.6 Sodium salt of alkyl aryl sulfonates 1.2 to 2.2

Parts of the above solids and liquids may vary from 42 parts to 70 parts to 400 parts of water.

A solution made up as taught in Example #1 produces a dense black oxide film on the conventional aluminum alloy as 28, 3S, 17S, 24S, 535, etc. To form an equivalent coating on a higher silicon and magnesium sand coating aluminum alloy as 214B which consists of 2% silicon, 5% magnesium and balance aluminum, the oxidizing agents must be increased about 3.5%; the hydrochloric acid lowered about 18.8%; with the sulphuric acid increased about 11.9%. Tests show this cooling has an electrical resistance of over 300 ohms when the solution is made according to Example #2 and operated at 175 degrees Fahrenheit for one minute.

The addition of the components of the various examples in correct sequence is important. If the hydrochloric acid is not added until after the other components, in Example #1, a muddy grey oxide film is produced instead of a clear dense black coating. The Wetting agent, or the sodium salt of alkyl aryl sulfonates, must be added as the last component in order to secure a uniform oxide on the aluminum. Example #1 and Example #2 solutions must have the components added in the following order:

1. Water 2. Potassium permanganate 3. Sulphuric acid 4. Sodium chromate 5. Hydrochloric acid The wetting agent The proper sequences of adding the components in both Examples 1 and 2 is necessary to create a bubbling action in the solution. If this bubbling action is not produced a spotty black oxide film is formed on the aluminum.

It is necessary to condition all of the solution made according to the examples before the production parts are started. This is done by merely adding some scrap aluminum which will cause a full chemical reaction.

The effective temperature of these various oxidizing solutions of this instant invention range from about 68 degrees Fahrenheit to about 220 degrees Fahrenheit. A solution made up as Example #1 produces a dense black film on aluminum parts when the solution is maintained between degrees to degrees Fahrenheit. A satisfactory black oxide film is formed on high silicon sand casting aluminum alloys when immersed in a solution made up as Example #2 and heated from 170 degrees to about 190 degrees Fahrenheit.

I have discovered a simple, inexpensive, conveyorized, mass production method of producing corrosion resistant films on aluminum. The steps of the process consist of the following:

1. Cleaning of the aluminum piece parts is the first step. The type of soil, dirt or contaminate determines the cleaning treatment. Sand or certain types of material such as found on cast parts do not effect the coating, if this foreign material is of such nature that the solution reaches the surface of the base aluminum. Conventional mechanical or chemical cleaning, degreasing, etching, will suffice in most cases.

2. Pre-treatment or passivation of the metal.

This step is optional in most instances, and may be carried out prior to the oxidation operation.

3. Immersing the piece part or continuous web in the particular oxidizing bath is the next step. The immersion time is dependent upon the type, weight or shape of the web or piece part that is to be coated. The aluminum part must reach the required temperature before the full benefit of the solution is achieved. Thirty (30) seconds have proved to be satisfactory for a thin coat. Increasing the immersion time is sometimes necessary to secure the desired finish. A three (3) minute immersion time maintaining the bath at 185 degrees Fahrenheit produces very satisfactory coatings on most aluminum parts.

4. Dipping of the aluminum part or web in a rinse solution is the fourth step. The object of this rinse is to wash 01?, as much as possible, the above mentioned solution. This rinse water may or may not contain additives to neutralize the residual solution, to passivate or to activate the oxidized film. Time of rinsing or temperature of the rinse bath is governed by the conditions required.

5. Immersing the rinsed object in a second rinse may be necessary to remove the excess of the preceding solution. Conditions of the part immersed will govern the time and temperature conditions.

6. Immersing the aluminum parts in a bath consisting of materials which impregnate the pores of the film may be carried out. This aftertreatment is used to fill the pores of the film so as to present an unbroken surface. A continuous film is desired to prevent very severe corrosion, to increase abrasion resistance and to secure other characteristics such as pan release, etc. l

The temperature of the bath and immersion time is controlled by the properties desired. This step is eliminated in most cases.

7. The next step consists in drying the aluminum coated objects by conventional means such as blowing with hot air and the like.

Tanks for the aluminum oxidizing solutions disclosed in this application should be constructed from stainless steel or be rubber lined. Agitators, fixtures or any parts to be immersed in the chemical oxidizing solution should be constructed of aluminum or stainless steel. Conventional exhaust systems may be installed on the oxidizing tanks. Air or mechanical agitators may be used to keep the solution stirred.

Chemical action between the solutions and the aluminum can be noted by the rise to the surface of the solution, of bubbles and products of the chemical reaction. This evolution of bubbles in the black producing solution begins as soon as the components are mixed in the proper proportions and sequence, even before any aluminum parts are treated. This bubbling action may continue for as long as several months after the solution has been stored.

The films developed by this instant invention possess good corrosion resistant. Salt spray tests were run by the standard Army-Navy aeronautical test No. ANQQ-S-91. The black film held up under the above salt spray for fifty (50) to seventy-five hours. In the quality of resistance to corrosion, the coating made by the processes of this invention may be said to be generally superior to that of the prior nonelectrolytic coating.

Films made by this novel process exhibit high dielectric strength. Using an ohm-meter, up to eighty thousand ohm resistance is detected. This means that a resistance is noted before a break-through of the contacts of the ohm-meter through the chemically oxidized film produces zero reading. It is estimated that 0.11 volt is exerted when a resistance of three hundred ohms is noted. Likewise a reading of two hundred fifty thousand ohms reading denotes a voltage of 0.3 using an ohm-meter of the type manufactured by Triplet Co. of Findlay, Ohio.

The films described in this application withstand elevated temperatures. Samples have been maintained at five-hundred degrees Fahrenheit for several hours without any noticeable effects.

The hard, dense films produced by the bath described in this disclosure exhibit good abrasion resistance. The film's resistance to normal abuse from sharp objects is good. Due to the formation of the various oxide or salt compounds formed on the surface of the aluminum, good protection from tarnishing by chemicals, marking from finger prints and ability to withstand outdoor weather conditions is noteworthy.

Usually a sealing operation is not necessary in producing a satisfactory film in this invention. Sufficient salt spray and abrasion resistance have been found in a great many cases, eliminating the need of this operation. If this added protection is deemed necessary, the conventional method of sealing may be used such as a hot solution of either potassium dichromate, sodium chromate, sodium silicate, nickel or cobalt acetate, boiling water, combinations thereof and the like.

This novel oxidizing finish may be produced on practically all types of aluminum. Alloys containing high percents of silicon of as much as 13.0% have been chemically oxidized by this process. Aluminum alloys high in silicon, magnesium or copper have caused considerable trouble when anodized or chemically oxidized by conventional processes, yet these alloys may be treated by this invention successfully.

The dense hard adherent coating produced by the instant process is resistant to abrasion and will not crack or peel off when the treated article is bent. This toughness of the films produced by this invention are not commonly found in the properties of other aluminum oxide coating.

This coating as produced on alloys of diiferent chemical analysis of aluminum, acts as a very good base for subsequent finishing coats such as paint, lacquer, enamel, etc. Since this invention can produce a coating on unusual shapes and forms of aluminum parts, which is impossible by conventional anodizing processes utilizing electric current, many advantages are gained.

The instant process produces an aluminum article with an oxide-like film'that can be welded or brazed.

The'coating may, however be used for decorative efiect, since it has a very pleasing appearance. It may also be employed as an absorbent for lubricants, where two objects are subjected to friction through movement. For this purpose the coating may be impregnated with a material such as oil, graphite or wax. The coated aluminum part is immersed into the molten or emulsified impregnating materials. The treated aluminum objects are allowed to dry leaving the impregnating materials in the pores or on the surface.

This new process may be easily applied to both webs and wires of aluminum. This inexpensive application of a protective film to aluminum by thenovel process is a boon to various industries such as the motor manufacturer using aluminum wire, aluminum clothes line manufacturer, aluminum siding or roofing converters, aluminum nail makers or others utilizing any or the multitude of applications or" aluminum. Unusual objects of aluminum such as zipper scoops attached to the cloth web may be coated by this continuous oxidizing process easily and at a lower cost than the conventional means.

This chemical oxidization process utilizing chemicals without any form of applied electric current, has been used with equal success on magnesium as well as aluminum.

According to the provisions of the patent statutes, I have described the principle and characteristics of my invention together with the preferred manner of practicing it and several variations and modifications thereof. I desire to have it understood that, within the scope of the appended claims, the invention may be practiced ctherwise than specially described herein.

This application is a continuing application of my application Serial No. 199,361, Methods for Producing Protective Films on Aluminum, filed December 5, 1950, now abandoned.

I claim:

1. An aqueous acid solution for the treament of aluminum and alloys thereof to produce oxide coatings, containing as essential active coating ingredients 26.0% to 30.6% of sodium chromate, 9.4% to 22.4% of sulphuric acid, 14.8% to 34.6% of hydrochloric acid, 26.5% to 30.6% of potassium permanganate and 1.2% to 2.2% of water soluble salts of allryl aryl sulfonates, the ratio of the total parts of ingredients to water being in the range between 42 parts and 70 parts per 400 parts of water.

2. The method of producing an oxide coating on aluminum and alloys thereof, which comprise immersing the aluminum objects in an aqueous acid solution maintained at a temperature in the range between about 68 degrees Fahrenheit and about 220 degrees Fahrenheit for a period of about 30 seconds to about 3 minutes and containing as its essential active coating ingredients about 1.2% to about 2.2% of water soluble sodium salts of alkyl aryl sulfonates and sulfate ions, chromate ions, chloride ions, and manganese ions in quantities produced in acid solution by about 9.4% to about 22.4% of sulphuric acid, 26.0% to about 30.6% of sodium chromate, about 14.8% to about 34.6% of hydrochloric acid and 26.5% to about 30.6% of potassium permanganate, the total of said ingredients being in quantities ranging from about 42 parts to about 70 parts by weig t of solids to about 400 parts of water.

3. An aqueous acid solution for treatment of aluminum and alloys thereof to produce oxide coating containing as essential active coating ingredients, 27.1% of sodium chromate, 10.8% of sulphuric acid, 33.6% of hydrochloric acid, 27.1% of potassium permanganate and 1.4% of sodium salt of alkyl aryl sulfonates.

4. An aqueous acid solution for the treatment of alumnium and alloys thereof to produce oxide coating containing as essential active coating ingredients, 30.6% of sodium chromate, 22.4% of sulphuric acid, 14.8% of hydrochloric acid, 30.6% of potassium permanganate and 1.6% of sodium salt of alkyl aryl sulfonates.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,042,611 Lukens June 2, 1936 2,224,528 Sutton et al Dec. 10, 1940 2,276,353 Thompson Mar. 17, 1942 2,352,076 Bushrod June 20, 1944 2,438,877 Spruance Mar. 30, 1948 2,465,443 Gide Mar. 29, 1949 2,502,441 Dodd et al Apr. 4, 1950 2,507,956 Bruno et a1. Mar. 16, 1950 2,512,493 Gide June 20, 1950 2,548,420 Chester et al Apr. 10, 1951 FOREIGN PATENTS Number Country Date 586,517 Great Britain Mar. 21, 1947 OTHER REFERENCES Mellors Modern Inorganic Chemistry revised by G. D. Parkes (1939), page 772; Longmans, Green and Co., New York. 

2. THE METHOD OF PRODUCING AN OXIDE COATING ON ALUMINUM AND ALLOYS THEREOF, WHICH COMPRISE IMMERSING THE ALUMINUM OBJECTS IN AN AQUEOUS ACID SOLUTION MAINTAINED AT A TEMPERATURE IN THE RANGE BETWEEN ABOUT 68 DEGREES FAHRENHEIT AND ABOUT 220 DEGREES FAHRENHEIT FOR A PERIOD OF ABOUT 30 SECOND TO ABOUT 3 MINUTES AND CONTAINING AS ITS ESSENTIAL ACTIVE COATING INGREDIENTS ABOUT 1.2% TO ABOUT 2.2% OF WATER SOLUBLE SODIUM SALTS OF ALKYL ARYL SULFONATES AND SULFATE IONS, CHROMATE IONS, CHLORIDE IONS, AND MANGANESE IONS IN QUANTITIES PRODUCED IN ACID SOLUTION BY ABOUT 9.4% TO ABOUT 22.4% OF SULPHURIC ACID, 26.0% TO ABOUT 30.6% OF SODIUM CHROMATE, ABOUT 14.8% TO ABOUT 34.6% OF HYDROCHLORIC ACID AND 26.5% TO ABOUT 30.6% OF POTASSIUM PERMANGANATE, THE TOTAL OF SAID INGREDIENTS BEING IN QUANTITIES RANGING FROM ABOUT 42 PARTS TO ABOUT 70 PARTS BY WEIGHT OF SOLIDS TO ABOUT 400 PARTS OF WATER. 